Heretofore, it has been known to temper a glass sheet by heating the glass sheet to a temperature close to its softening point and rapidly quenching the surfaces of the glass sheet heated close to the softening point. A glass sheet can be made to have a larger compression stress imparted to its surfaces by increasing the temperature difference between the center portion in the thickness direction and the surfaces of the glass sheet. Therefore, it is effective to increase the temperature difference between the center portion and the surfaces of a glass sheet in order to temper the glass sheet or to control fragments (spline) when ruptured.
Here, as a means to increase the temperature difference between the center portion and the surfaces of a glass sheet, an invention described in Patent Document 1 discloses a method of heating a glass sheet by means of radio-frequency energy. Patent Document 1 discloses a method and apparatus for producing a tempered glass sheet, wherein a glass sheet is heated by means of radio-frequency energy formed by a feed electrode and a passive electrode provided in a pair so that the glass sheet is placed therebetween, and at the same time, the surfaces of the glass sheet are cooled to increase the temperature difference between the center portion and the surfaces of the glass sheet, followed by quenching to obtain a tempered glass sheet.